The present invention generally relates to synchronizing signal separating circuits for recording and reproducing apparatuses which record and reproduce time-division-multiplexed color video signals, and more particularly to a synchronizing signal separating circuit for a recording and reproducing apparatus which subjects a time-division-multiplexed color video signal to a frequency modulation and records the frequency modulated signal on a recording medium. The time-division-multiplexed color video signal is obtained by subjecting a luminance signal and a line-sequential color difference signal to a time base compression, and time-division-multiplexing the time base compressed signals together with an achromatic level signal. At the time of a reproduction, the recording and reproducing apparatus frequency-demodulates the frequency modulated signal which is reproduced from the recording medium, and subjects the frequency demodulated signal to a time base expansion so as to obtain a reproduced color video signal. The synchronizing signal separating circuit according to the present invention satisfactorily separates a synchronizing signal within a reproduced time-division-multiplexed color video signal which is obtained by the frequency demodulation carried out in the recording and reproducing apparatus at the time of the reproduction.
Among the existing color video signal recording and reproducing apparatuses such as video tape recorders (VTRs), the more popular recording and reproducing apparatuses separate a luminance signal and a carrier chrominance signal from a standard system composite color video signal. The standard system is a system such as the NTSC system, the PAL system, and the SECAM system. The separated luminance signal is frequency-modulated, and the separated carrier chrominance signal is frequency-converted into a low frequency range. The frequency converted carrier chrominance signal is frequency-division-multiplexed with the frequency modulated luminance signal and recorded on a recording medium. At the time of the reproduction, a signal processing opposite to the signal processing carried out at the time of the recording, is carried out to obtain a reproduced composite color video signal which is in conformance with the original standard system. In other words, the more popular recording and reproducing apparatuses employ the so-called low-band-conversion recording and reproducing system.
Other various recording and reproducing systems have been proposed besides the low-band-conversion recording and reproducing system. For example, there was a proposed recording and reproducing apparatus which was designed to subject two kinds of color difference signals which are obtained by frequency-demodulating the carrier chrominance signal to a time base compression, and also subject the luminance signal to a time base compression. According to this proposed recording and reproducing apparatus, the time base compressed signals are time-division-multiplexed, and the time division multiplexed signal is frequency-modulated and recorded on the recording medium. At the time of the reproduction, a signal processing opposite to the signal processing carried out at the time of the recording, is carried out to obtain a reproduced composite color video signal which is in conformance with the original standard system. An example of such a recording and reproducing apparatus may be found in the U.S. Pat. Nos. 3,781,463 and 4,245,235, for example. This proposed recording and reproducing apparatus takes into account the difference in the bands of the luminance signal and the color difference signals, and takes measures so that the color difference signals having the narrower band can be transmitted within the horizontal blanking period. In other words, one of the color difference signals which is transmitted within one horizontal scanning period (1H), is subjected to a time base compression into approximately 20% of 1H. In addition, to utilize the band effectively, the luminance signal is subjected to a time base compression into approximately 80% of 1H so as to occupy a band which is in the same range as the band of the time base compressed color difference signal, and transmitted. Further, the two color difference signals are time-division-multiplexed, as a line-sequential signal in which the two color difference signals are alternately transmitted for every 1H, with the time base compressed luminance signal. This time-division-multiplexed signal is supplied to a frequency modulator, and an output signal of the frequency modulator is recorded on the recording medium. At the time of the reproduction, a signal processing opposite to the signal processing carried out at the time of the recording, is carried out to obtain a reproduced composite color video signal. The recording and reproducing system employed in this proposed recording and reproducing apparatus, will hereinafter be referred to as a timeplex system.
According to the timeplex system which transmits the time-division-multiplexed signal, there is no period in which the luminance signal and the color difference signal are transmitted simultaneously. In the case of the NTSC system color video signal and the
system color video signal, a mutual interference and moire may occur between the luminance signal and the color difference signals, because the luminance signal and the carrier chrominance signal are band-share-multiplexed and transmitted. However, such a mutual interference and moire will not occur according to the timeplex system. In addition, even when the color video signal of any one of the NTSC system, the PAL system, and the SECAM system is recorded by an azimuth recording and reproducing system on tracks having the horizontal synchronizing signals recorded in non-alignment between mutually adjacent tracks and then reproduced, there is substantially no crosstalk from the adjacent tracks due to the azimuth loss effect, and it is possible to obtain a reproduced picture of a high picture quality. This is because the time-division-multiplexed signal is recorded on the adjacent tracks in the form of a frequency modulated signal which is obtained by frequency-modulating by the time-division-multiplexed signal a high-frequency carrier which has a large azimuth loss effect.
The time base compressed luminance signal and the time base compressed color difference signal employed in the timeplex system, both have an energy distribution in which the energy is large in the low frequency range and the energy is small in the high frequency range. In other words, the time base compressed luminance signal and the time base compressed color difference signal assume a signal format which is suited for the frequency modulation. Thus, it is possible to obtain a large modulation index, and the signal-to-noise ratio can be greatly improved. Moreover, it is possible to substantially eliminate a deviation in the reproducing time base when expanding the time base.
In the conventional recording and reproducing apparatus which employs the timeplex system, the horizontal synchronizing signal is separated from the reproduced time-division-multiplexed color video signal at the time of the reproduction, in a synchronizing signal separating circuit. The reproduced horizontal synchronizing signal which is separated in the synchronizing signal separating circuit, is differentiated to obtain a pulse. A time position of this pulse is used as a reference to separate the reproduced time base compressed luminance signal and the reproduced time base compressed line-sequential color difference signal from the reproduced time-division-multiplexed color video signal. Further, the time position of this pulse is also used to determine a starting point of the write-in and read-out into and from a memory circuit for the purpose of performing the time base expansion. Accordingly, the separation of the horizontal synchronizing signal plays an important role in the recording and reproducing apparatus which employs the timeplex system.
Generally, the synchronizing signal separating circuit is designed to slice the horizontal synchronizing signal at approximately one-half the peak value of the horizontal synchronizing signal, and to form a horizontal synchronizing signal detection pulse at the point in time when the slicing takes place, in order to avoid the effects of noise or the like which mixes into the horizontal synchronizing signal. The horizontal synchronizing signal detection pulse is produced from the synchronizing signal separating circuit as a separated horizontal synchronizing signal. Hence, it is possible to accurately separate the horizontal synchronizing signal without being affected by noise which is mixed at the synchronizing tip level position and the pedestal level position.
However, depending on the recording and reproducing characteristics or the like, the waveform of the reproduced horizontal synchronizing signal may become distorted or different from the original waveform of the horizontal synchronizing signal. Moreover, the deviation of the waveform of the reproduced horizontal synchronizing signal from the original waveform, is not constant. In other words, the slope of the leading edge of the reproduced horizontal synchronizing signal is sometimes small and sometimes sharp. For this reason, in the conventional synchronizing signal separating circuit, there are cases where the time it takes for the reproduced horizontal synchronizing signal to reach substantially one-half the peak value from the starting time of the horizontal synchronizing signal, is not constant. In these cases, it is impossible to perfectly separate the reproduced time base compressed luminance signal and the reproduced time base compressed line-sequential color difference signal, and a dropout may occur in a part of the reproduced color difference signals and the reproduced luminance signal. In addition, because the time base expansion ratios are different for the reproduced time base compressed luminance signal and the reproduced time base compressed line-sequential color difference signal, a relative time deviation is introduced between the reproduced luminance signal and the reproduced color difference signals which are obtained by the time base expansion, due to the imperfect separation of the horizontal synchronizing signal in the synchronizing signal separating circuit. As a result, there is a problem in that each color information are not displayed at the correct positions in the reproduced picture due to the above relative time deviation.